It has been recognized for many years that medium- and low-strength materials such as plastics, aluminium, etc. can be reinforced by fibres (often referred to as "whiskers") and/or particulates of strong materials such as silicon carbide. Whiskers are small single crystal fibres which can be mixed with, for example, a powdered metal or plastic to form a composite material. The whiskers have extremely high values of strength, elastic modulus, heat resistance and chemical stability, and can be used to reinforce and greatly improve the mechanical properties of other materials. In addition, it is also possible to use such whiskers in order to achieve specific physical and chemical properties, for example controlled thermal and electrical conductivity.
Metal matrix composites are materials which comprise a metallic phase consisting of a matrix of a metal or an alloy, and generally up to about 50% by volume of strong reinforcing fibres and/or particles. When the fibres and/or particles are primarily used for the purpose of improving the mechanical properties of the metal matrix composite, they will generally be present in an amount of not more than about 30% by volume, while amounts greater than 30% by volume may be employed when the fibres and/or particles are, for example, used in order to improve other physical properties of the matrix. The reinforcing fibres or particles are composed of materials which are significantly stronger than the metal or alloy of the matrix, and the metal matrix is thereby strengthened to an extent depending upon the strength of the reinforcing fibres or particles and the amount of reinforcing fibres or particles included in the composite material. Such fibres or particles normally have a diameter of about 0.1-5 .mu.m and a ratio between length and diameter of, for example, about 1-100. These metal matrix composite materials are primarily fabricated by powder metallurgical techniques, fusion metallurgy or diffusion bonding, followed by secondary fabrication involving conventional metal-working processes such as extrusion, cold-rolling or drawing.
Aluminium based metal matrix composites are of great interest, in particular in the automotive and aerospace industries, due to their relatively high specific strength and low weight. The aluminium or aluminium alloy matrix in such materials can be reinforced by means of continuous or short fibres, whiskers or particles which have a much higher strength and elastic modulus than the matrix.
While the advantages of using fibres of, for example, SiC are theoretically well defined, it has been very difficult to properly mix the fibres with other substances due to their pronounced tendency to agglomerate. The agglomeration of the SiC fibres which occurs is believed to be caused by electrostatic forces arising from the build-up of charges on the surface of the fibres. SiO groups also occur on the surface of the fibres, and it is believed that this results in the formation of SiOH groups, which in turn results in hydrogen bonding between the individual fibres and thus contributes to their agglomeration. When mixed with other materials, the agglomerated state of the fibres will lead to the formation of a non-homogeneous composite of lower mechanical strength than that of an otherwise equivalent material with uniformly distributed fibres.
In order to achieve the maximum benefits of fibre reinforcement, it is necessary to thoroughly and uniformly disperse the fibres in the material. Such a dispersion requires thorough deagglomeration and mixing without any substantial damage to the fibres.
In U.S. Pat. No. 4,632,794 and U.S. Pat. No. 4,634,608, it is taught that an agglomerated whisker mass can be deagglomerated prior to mixing by treating it with a polar solvent, e.g. water or a polar organic solvent such as an alcohol or a ketone. It is especially interesting to note that the deagglomeration treatment is so crucial and so difficult that the above-mentioned U.S. Pat. No. 4,634,608 describes a complex two-stage procedure in which the whiskers are first mixed in one polar solvent to form a slurry, which is then dried, after which the whiskers are mixed with another polar solvent. It is claimed that a better deagglomeration is obtained with this two-step procedure, but even with this method, the results do not appear to be completely satisfactory, since the specification recommends that the second slurry be sieved in order to remove any agglomerates which are still present. This is also the case with the process according to the above-mentioned U.S. Pat. No. 4,632,794, which includes a step in which the dispersion of whiskers is passed through a sieve to separate out any insufficiently dispersed whisker agglomerates.
In the preparation of objects based on a metal matrix composite, as-supplied composite billets are often exposed to high temperatures for remelting and casting into a near-net shape, in other words a shape which is almost the final shape of the finished object. Metal matrix composite objects prepared by casting are also subject to high temperatures during the casting process. These processes may however lead to severe chemical attack at the reinforcement-matrix interface, and the development of metal matrix composites has to a certain extent been limited by these high-temperature reactions which degrade the reinforcement. This is for example the case with unprotected SiC whiskers, which are thermodynamically unstable in most aluminium alloys. The main reaction between liquid aluminium and SiC is: EQU 4 Al+3 SiC.fwdarw.Al.sub.4 C.sub.3 +3 Si.
As explained in detail below, it has been found according to the invention that it is possible to provide reinforcing fibres and/or particles, e.g. SiC whiskers, with a protective coating which may serve to reduce chemical reaction at the interface between the fibres and/or particles and the metal or alloy of the matrix.